DVD's (Digital Versatile Disc) that have approximately seven times the capacity of CD's (Compact Disc) have rapidly gained in popularity in recent years. Further, DVD-videos that are capable of being reproduced in high-volume are replacing VHS and the like, tape mediums used as mediums for renting and distribution of contents of movies and other material.
Moreover, standards for recording such as DVD-RAM, DVD-R, DVD-RW, DVD+R and DVD+RW are becoming commonplace for PC drives and video recorder units.
Recordable CD-R is already commonly used.
In light of the above, recording functions for either the 650 nm band for DVD or the 780 nm band for CD are required in an optical disk recording device.
Further, compatibility in both recording and reproduction is needed for all of the various standards used for DVD, however the structure and functions of an optical pickup used for this are complex.
The need for such devices to be inexpensive, small and lightweight are increasing due to the demands of the general population, thus development of a simple, compact and inexpensive optical pickup, that is nonetheless complex and replete with multiple functions, is required.
An optical pickup furnishing the above functions is called a two wavelength recording pickup.
Generally, such a two wavelength recording pickup must fulfill the following requirements.
1. Optical System Dependent on Polarization-Optical System Independent of Polarization
In a DVD recording type pickup, due to the combining of a PBS (polarized light beam splitter) and a wavelength plate (in a polarized light system) the efficiency of outward path (i.e. from the source) and return path should approach 100 percent and recording power must be maintained while the load on the laser light source is decreased.
In the case of a CD system the load on the laser light source is not so great, further a large number of CD disks with substantial birefringence are circulating in the market. Accordingly, in the CD line, polarized light independent systems have become the practical standard due to the need to avoid the side effect of deteriorating replay.
2. Beam Shaping
In order to effectively utilize the elliptical shaped beam intensity distribution of laser emitted light, normally in the case of DVD recording optical systems, a method is used in which a wedge shaped transmissive part is inserted making the intensity distribution round (beam formation). Beam formation is especially important in the case of DVD-RAM where high recording power is required. Limitations applying to beam formation include 1) it must be performed in parallel light beams and 2) beam formation in two wavelengths simultaneously in the same prism is difficult due to chromatic aberration.
In the case of CD systems the above described load on the laser source is light and such beam formation is unnecessary.
FIG. 1 provides an example of a conventional two wavelength recording pickup that fulfills the above described requirements in outline.
As shown in FIG. 1, a DVD laser light source 601, a collimator lens 602, a grating 603, a front monitor 604, a polarized light beam splitter 605, a ¼ wavelength plate 606, a dichroic mirror 607, a mirror 608, a secondary collimator lens 609, a detecting lens 610, light receiving elements 611, an integrated device providing a CD laser light source 612, a collimator lens 613, a mirror 614 and a secondary front monitor 615. The polarized light beam splitter 605 provides the above described function for intensity distribution formation.
After being emitted from the CD laser light source 612, CD laser light of the above described pickup is collimated at the collimator lens 613 passing via the mirror 614 (without undergoing beam formation) and is directed to an optical disk not shown in the drawing. Following the same path, returning light from the optical disk returns to the receiving elements inside the laser light source 612.
DVD laser light is emitted from the DVD laser light source 601 as P polarized light waves and after being made parallel at the collimator lens 602, passes via the grating 603 and is injected from an end 605a of the polarized light beam splitter (PBS) 605 before being reflected at a reflecting surface 605b. Thereafter, this DVD laser light passes via a PBS film surface 605d and is emitted from the other end 605c, before being formed into circular polarized light at the wavelength plate 606, attached so as to be in contact with the end 605c. The laser light is then directed to an optical disk not shown in the drawing. Returning light from the optical disk is made into S polarized light at the wavelength plate 606 and is reinjected into the end 605c of the polarized light beam splitter 605, reflected at the PBS film surface 605d of the polarized light beam splitter 605 (return path optical system is separated) and reaches the light receiving elements 611 via the detecting lens 610, after being collimated at the secondary collimator lens 609.
However, this pickup has separate respective collimator systems for the outward path and return path at the DVD side for example and as there are basically no common parts of the CD side and DVD side, the structure becomes complex as a large number of parts are required notwithstanding the integrated device.
FIG. 2 and FIG. 3 relate to an example of an optical pickup (Japanese Patent Application Laid-Open No. 6-325405) similar to the conventional optical pickup described above. This pickup is what is known as a “combo drive” optical pickup, capable of recording only on the CD side. To allow for cases when the output of the laser light source that outputs 780 nm band laser light is insufficient, this optical pickup has a beam forming means for 780 nm band laser light and is an optical system dependent on polarization.
That is to say, the CD laser light is emitted from a light source 702, passes via a collimator lens 712 and undergoes beam formation at a prism 713. The laser light is directed to a disk 709 passing via beam splitters 705 and 706, a wavelength plate 707 and objective lens 708. The return path light from the disk 709 passes via the wavelength plate 707 and the beam splitter 706 and is injected into the beam splitter 705. Due to the PBS properties of this beam splitter 705, the optical path of the CD laser light changes to the side having this collimator lens 704. This return path light undergoes a further optical path conversion at a PBS 703, returning to light receiving elements 711 after passing via a detecting system lens 710.
In the case of the DVD laser light, this light is emitted from a light source 701 and returns to the beam splitter 705 via the PBS 703 and collimator lens 704. Here, as shown in FIG. 3, the beam splitter 705 operates to reflect short wavelengths (λ1) and as a PBS for long wavelengths (λ2). Accordingly, DVD laser light from the light source 701 is reflected at the beam splitter 705, moreover is directed to the disk 709 after passing via the beam splitter 706, the wavelength plate 707 and the objective lens 708. Return path light from the disk 709 returns to the PBS 703 along the same path, passing via the objective lens 708, the wavelength plate 707, the beam splitter 706, the beam splitter 705 and the collimator lens 704. This return path light is separated at the PBS 703 and reaches the light receiving elements 711 passing via the detection system lens 710.
The PBS 703 is used to branch the outward and return optical paths in this optical pickup. Accordingly, the optical axis of emitted light and the optical axis of received light are disposed mutually separated at approximately 90 degrees such that concentration of the light receiving parts is not practically possible.
Moreover, the two wavelengths of the CD laser light and the DVD laser light are optical systems dependent on polarization, accordingly there is concern of deterioration in the replay performance of a CD disk having substantial birefringence.
FIG. 4 shows another example of an optical pickup, being that disclosed in Japanese Patent Application Publication Laid-Open No. 10-334500.
As shown in FIG. 4, this optical pickup is a replay pickup device using an integrated devices 801 and 802 that emit and/or receive light for the respective two wavelengths and having a prism 803 for separating and synthesizing the optical paths of the two wavelengths inserted in divergent light, such that the collimator lens 804 is shared.
That is to say, a laser beam emitted from a laser chip 805 of the primary integrated device 801 is injected into a wedge shaped prism 803 while diverging and after being reflected at the surface of the prism 803 enters a collimator lens 804. The primary laser light, formed into parallel light beams by this 804, passes via an aperture stop 806 and from an objective lens 807, is focused on the signal recording surface of the disk 709. Primary laser light reflected at this disk 709 returns to the prism 803 passing via the objective lens 807, the aperture stop 806 and the collimator lens 804. This primary laser light is reflected at the surface of the prism 803 and is received at light receiving elements 809 after passing via a hologram 808 of the primary integrated device 801.
On the other hand the secondary laser light emitted from a laser chip 810 of the secondary integrated device 802 and having a wavelength which differs to that of the primary laser light is injected into the wedge shaped prism 803 while diverging and, passing through this prism 803 enters the collimator lens 804. This secondary laser light, formed into parallel beams at the collimator lens 804 passes via the aperture stop 806 and is focused on the recording surface of the disk 709 by the objective lens 807. Secondary laser light reflected at this disk 709 returns to the prism 803 passing via the objective lens 807, the aperture stop 806 and the collimator lens 804. Passing through the prism 803, the secondary laser light is received at the light receiving elements 812 after passing a hologram 811 of the secondary integrated device 802.
The object of this optical pickup also is the utilization of integrated device, however two integrated devices are required and the same problem of requiring a complex structure persists. Further, this optical pickup is for reproduction and does not provide an optical system that more efficiently utilizes optical properties such as by beam formation and a polarized light system, thus even if high output laser is used this structure does not actually enable recording to an optical disk.
Moreover, as shown in FIG. 5, wavelength dependency of the polarized light beam splitter film appears to be used in the prism 803 of this optical pickup, but actually in the 650 nm band (the primary laser light) total reflection arises in a system independent of light polarization while in the 780 nm band (the secondary laser light) all the light passes in a system independent of light polarization, therefore this polarized light beam splitter film simply functions as a dichroic mirror.
A noticeable characteristic of this optical pickup is substantial dependence of the operation of the wavelength selecting film on angle of incidence, the optical pickup providing a film design in which, within a range of angles of divergent light, sufficient dichroic properties are realized independent of polarization. Further as the prism has a wedge angle, aberration occurring when a planar member is inserted in divergent light are cancelled out.
Accordingly, this optical pickup does not use polarized light for separation of two wavelengths, while the optical pickup can be applicable to a polarized light hologram.
Further, two integrated devices are necessary as a result of the progression towards integration and this leads unavoidably to complexities in structure and increased production costs.
When integration and miniaturization are pursued overall in a recording type optical pickup device in the case of a conventional optical pickup as described above, the following problems occur due to the requirements of a system accommodating each of the different wavelengths as described above.
1. Efficiency of Outward and Return Optical Path Separating Elements
In the above described optical system dependent on polarization, efficiency is achieved in what is largely an ideal outward and return path by use of a polarized light beam splitter, but in the case of a CD system which is a polarization independent optical system such efficiency cannot be realized. Thus, a polarization independent beam splitter is used for outward and return path separating elements in a CD system, and efficiency distribution placing priority on light intensity required at the surface when recording is required, this being a return path efficiency (or permeation efficiency when the returning path light passes) of 60 percent to 90 percent.
2. Heat Generation and Integration
A recording type optical pickup requires a high output laser in the 100 mW to 200 mW class. Accordingly substantial power is consumed and a concomitant rise in temperature due to heat generation is unavoidable when recording is performed, in other words, when the laser is generating light at high output.
With a conventional laser diode in can package light source expelling heat generated by the laser chip is relatively simple. In contrast to this however, in an integrated device in which the light receiving elements or hologram elements are integrated, a plurality of parts intercede in the path of heat conduction thereby preventing sufficient heat release.
In other words, a complex structure having a plurality of parts results if satisfactory heat release is to be achieved. Realizing both a simple structure and adequate heat release is difficult especially when two wavelengths share common optical paths and where integrated devices are used for both wavelengths.